1. Field of the Invention
The present invention relates to an intake-and/or exhaust-valve timing control system for internal combustion engines which is variably capable of controlling the intake- and/or exhaust-valve timing depending upon the operating state of the engine, for example the magnitude of engine load and/or engine speed.
2. Description of the Prior Art
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance according to the operating state of the engine.
As is generally known, the valve timing is determined such that optimal engine performance is obtained, however the predetermined valve timing is not suitable under all operating conditions. For example, when the engine is operating within a range of low revolutions, higher torque will be obtained with an intake-valve timing earlier than the predetermined valve timing.
Such a conventional intake- and/or exhaust-valve timing control system for internal combustion engines has been disclosed in U.S. Pat. No. 4,231,330. In this conventional valve timing control system, a cam sprocket is rotatably supported through a ring gear mechanism by the front end of a camshaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. The ring gear is normally biased in the axial direction of the camshaft by means of a return spring, such as a coil spring. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and the cam sprocket (and consequently, the phase angle between the camshaft and the engine crankshaft) is relatively varied. The ring gear moves as soon as one of the two opposing forces acting on it, namely the preloading pressure of the above spring means or the oil pressure applied from the oil pump to the ring gear, exceeds the other. However, in this conventional valve timing control systems, each of the two meshing pairs of gears has backlash or play therebetween. During operation of the ring gear, the backlash results in collision between the teeth and thereby causes noise and fluctuations in the torque of the camshaft.
To avoid the above problem, an improved conventional intake- and/or exhaust-valve timing control system has been disclosed in Japanese Patent First Publication (Tokkai Showa) 61-279713. In this valve timing control system, the ring gear, which is disposed between the timing belt pulley and the camshaft, includes a pair of ring gear elements. The pair of ring gear elements are formed in such a manner as to divide a relatively long ring gear including inner and outer toothed portions into two parts by cutting or milling. Therefore, the two ring gear elements have essentially the same geometry with regard to the inner and outer teeth. These ring gear elements are interconnected by a plurality of connecting pins which are fixed on one of the ring gear elements through an annular hollow of the other ring gear element. The annular hollow is traditionally filled with elastic materials, such as cylindrical rubber bushing attached by vulcanizing. Alternatively, a plurality of coil springs may be provided in the annular hollow, while the springs are supported by the heads of the connecting pins serving as spring seats. In this conventional timing control system, when the ring gear elements and the connecting pins are assembled, the two ring gear elements are interconnected in such a manner as to be slightly offset from each other. In other words, the angular phase relationship between the two ring gear elements is designed so as to be set to an angular position slightly offset from each other. The above noted offset is preset to a slightly greater value than the offset of the ring gear when meshed with its connecting gears. In this construction of the ring gear, due to the offsetting of the ring gear elements, the apparent tooth thickness of each tooth of the ring gear is greater than the actual tooth thickness. Therefore, backlash between each meshing pair of gears is eliminated by the return spring force generated by the cylindrical rubber bushing or the coil springs serving as a backlash eliminator.
In the aforementioned constructions, the conventional valve timing control system disclosed in the Japanese Patent First Publication (Tokkai Showa) 61-279713 can effectively prevent noise due to collision between teeth and fluctuations in the torque of the camshaft.
However, the construction of the previously noted two split type ring gear member having a backlash eliminator is complicated. Furthermore, since an axial length of the conventional ring gear member is relatively great, the entire length of the valve timing system is increased. As a result, the overall engine size and engine weight become large. Therefore, the lay-out of the engine may be limited in the engine room.